The present invention relates to a method of determining the thermal profile of a drilling fluid in a well.
During drilling, which is the mud injected into the drill string of the well and which flows back through the corresponding annulus, undergoes great temperature variations. The fluid can encounter temperatures that can range from 2xc2x0 C. for deep offshore wells to more than 180xc2x0 C. for very hot wells. Many mud properties, such as rheology or density, depend on the temperature. Calculation of the pressure losses during drilling can therefore be improved if an estimation of the temperature profile in the well is known. It is therefore important to be able to predict the temperature profile in the flowing mud from well data and mud characteristics.
Measurement of the thermal profile of the fluid in a well under drilling would require complete instrumentation of the well, that is installation of evenly spaced out detectors in the drill string and in the annulus, allowing temperature measurement at various depths. However, installing such a measuring system entails too many constraints; only localized measurements picked up by devices mounted in the drill string allow knowing certain temperature points on the path of the drilling fluid.
In the face of this lack of data, analytic models based on heat transfer equations have been developed to evaluate the thermal profiles of the fluid along the well under drilling. Some of these analytic models are implemented in softwares and allow providing an estimation of thermal profiles from a data which may be difficult to obtain. Thus, knowing the characteristics of the site and of the drilling equipment, by giving a value of the temperature of the fluid at the well inlet, these softwares can predict the temperature profile of the drilling fluid.
However, a comparison between the results obtained with analytic methods and the measurements obtained in the field shows that there can be great differences. Furthermore, the complexity of softwares using numerical calculation methods makes real-time implementation thereof difficult.
On the other hand, a study of the bibliography on thermal models shows a similarity in the form of temperature profiles in most cases, which turns on three points: inlet temperature, outlet temperature and bottomhole temperature.
The aim of this study is thus to provide a method allowing real-time determination of a thermal profile in the mud from three measuring points available in the field, that is the injection temperature, the outlet temperature and the bottomhole temperature measured by a detector mounted on the string. The form of the profile between these three points is represented by a type curve representative of the thermal profiles in a well under drilling, estimated from physical considerations on thermal transfers in the well.
The method of determining the thermal profile of a drilling fluid circulating in a well under drilling according to the invention is defined by the successive stages as follows:
a) determining a general expression xcex81 for the thermal profile of the fluid within the drill string in the well and a general expression xcex82 for a thermal profile of the fluid in the corresponding annulus, using the heat propagation equation that takes into account a thermal profile of the medium surrounding the well,
b) measuring the temperature of the fluid at the well inlet, T1, at the well bottom, T2, and at the well outlet, T3,
c) modifying the expressions xcex81 and xcex82 to meet the temperature boundary conditions T1, T2 and T3,
d) determining the thermal profile of the drilling fluid as a function of the depth.
In order to obtain, in real time, a temperature profile with the method presented above, stages b), c) and d) can be repeated.
According to the method of the invention, in stage a), general expressions xcex81 and xcex82 can comprise unknown constants, and in stage c), it can be determined that expressions xcex81 and xcex82 meet the temperature boundary conditions T1, T2 and T3 by determining the unknown constants.
In order to determine a general expression xcex81 for the thermal profile of the fluid within the drill string in the well and a general expression xcex82 for a thermal profile of the fluid in the corresponding annulus, it is possible, according to the method of the invention, to use in stage a) the heat propagation equation that takes into account at least the thermal equation of the medium surrounding the well, the flow rate of the fluid and the balance of the thermal exchanges undergone by the fluid, the thermal exchanges comprising at least the exchanges between the ascending and descending drilling fluid, and/or to use the equation of heat propagation in a homogeneous medium on a cylinder of infinite height centered on the well, the cylinder comprising the drill string that guides the descending fluid and the annulus around said drill string, which guides the ascending fluid.
According to the method of the invention, general expressions xcex81 and xcex82 obtained in stage a) can be split up into several independent equations and, in stage c), it can furthermore be determined that the profiles and the derivatives of the thermal profiles of the fluid within the drill string and in the corresponding annulus are continuous.
The method according to the invention can notably be used to calculate the pressure drops of the drilling fluid circulating in a well under drilling, or in another application, to determine the zones of hydrate formation in the fluid during drilling.
In relation to the methods for determining the thermal profile of a drilling fluid in a well according to the prior art, the present invention notably affords the following advantages:
the temperature profile obtained is more accurate because it is determined from three drilling fluid temperature measurement points while keeping an analytic expression of the thermal profile between the measuring points which is physically justified,
since the temperature measurements are performed all the time, the method allows obtaining the temperature profile in real time and to observe the evolution thereof with time.